The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
In the construction and development of wells formed in subterranean formations, such as wells for the production of oil and gas, various operations are carried out that require the introduction of fluids of different types into the wellbore and/or into formation surrounding the wellbore.
In certain applications it may be desired to pump or introduce a thin fluid and then rely on some trigger to make the fluid more viscous or viscoelastic to perform its intended function.
Viscoelastic surfactants (VES) have been used in fluids for many well treatment operations. Not to be limited by theory, but these viscoelastic surfactant systems under proper conditions form long rod-like or worm-like micelles in aqueous solution. Entanglement of these micelle structures gives viscosity and elasticity to the fluid. When these fluids are subjected to shear, the shear disrupts the micelle network so that the fluids exhibit shear-thinning behavior. When the shear is removed or lessened, the viscosity of the fluid recovers as the VES micelle network reassembles.
This shear-thinning behavior is beneficial in many well treatments. In hydraulic fracturing, for example, shear-thinning behavior is desired as the fracturing fluid is being pumped into the wellbore. Once the fracturing fluid enters the formation, however, increased viscosity is desired to facilitate suspension of proppant into the formed fractures. Thus, in these VES fluids the reduced shear results in an increase in viscosity of the fluid.
In coiled tubing cleanouts, it is desirable to introduce a thin fluid to minimize pumping power requirements, but then provide a thicker fluid once the fluid is jetted from the coiled tubing nozzle to suspend and transport particles back to the surface to facilitate cleanout. In this case, the VES fluid is thinned during the high shear imparted as the fluid is being pumped down the wellbore tubulars. Once jetted, the viscosity of the fluid recovers to facilitate cleanout.
U.S. Pat. No. 7,290,615 describes an example of a VES fluid that may be used for cleanout applications and that may be recycled for repeated use. In such applications, the pH of the fluid is cycled by utilizing pH adjusting agents to provide higher or thinner viscosity. Thus, when the VES fluid is introduced into the wellbore for cleanout, the fluid is provided with a high pH that provides a high viscosity for suspending particles cleaned from the well. When the cleanout fluid with the entrained particles is brought to the surface, the pH of the fluid is lowered so that the viscosity of the fluid is decreased to facilitate settling of the entrained particles. After the particles have settled, the fluid viscosity can then be increased again by readjusting the pH. The fluids may thus be used repeatedly by cycling the pH to different levels.
As can be seen, while the shear-thinning behavior of these VES fluids is beneficial, there may be applications where increases in viscosity of the VES fluid are necessary or beneficial in high shear environments or where an increase in shear may trigger an increase in viscosity without requiring chemical modification of the fluid. The present embodiments are directed to providing such fluids and methods.